The present invention relates to a plate mounted state confirming apparatus for a printing press, which confirms whether or not a plate is mounted on a plate cylinder with a high precision.
In each of various types of printing presses, leading- and trailing-side plate lockup devices are provided in a gap formed in the outer circumferential surface of a plate cylinder. When the leading end of a plate is gripped by the leading-side plate lockup device, the plate cylinder is rotated by almost one revolution, thereby winding the plate on the circumferential surface of the plate cylinder. Thereafter, the trailing end of the plate is gripped by the trailing-side plate lockup device, and the plate is mounted on the plate cylinder.
In the printing operation, an ink and water are supplied to the surface of the mounted plate to form an image, and the image is transferred to paper being transported directly or through a blanket cylinder, thereby printing the image.
In this printing operation, when the plate is distortedly mounted, the image is distortedly printed, degrading the quality of the printed matter. Especially, in multicolor printing, the different colors are misregistered to largely degrade the quality of the printed matter.
For this reason, conventionally, U-shaped reference pin holes are formed in the leading end face of the plate with reference to reference holes formed in the plate during plate making. The plate is mounted while engaging the reference pin holes with reference pins provided on the gripper portion of the leading-side plate lockup device.
More specifically, the gripper plate of the leading-side plate lockup device is opened, the leading end of the plate is inserted in the gripper portion of the gripper plate, the U-shaped bottom portions (insertion end) of the reference pin holes are abutted against the reference pins, this abutted state is confirmed according to the visual observation of the operator, and the gripper plate is closed.
However, with this plate mounting method, since abutment of the reference pin holes with the reference pins is confirmed according to the visual observation of the operator, variations are caused depending on different operators, and satisfactory positioning precision cannot always be obtained.
Therefore, recently, the present applicant proposed a method of confirming abutment of the reference pin holes against the reference pins. According to this method, the reference pins serve as electrodes. Contact of the insertion end with the reference pins is detected, and a detection result is obtained in the form of an electrical signal. This electrical signal is received outside the plate cylinder (at an external portion of the plate cylinder) through a signal transmitting means having a contact type contact structure employing, e.g., a brush scheme. A display is turned on based on the received electrical signal.
With this method, however, since the signal transmitting means for receiving the electrical signal has the contact type contact structure, incomplete contact tends to occur due to a degradation in conductivity or the like caused by an oxide formed by electric spark.